Important criteria for judging friction linings for clutches, in particular for motor vehicle clutches, are
the coefficient of friction .mu., PA1 the wear, PA1 the mechanical strength and fracture toughness, PA1 and the rotational speed at which rupture occurs, PA1 the tendency to self-induced frictional vibrations ("bucking" or "grabbing") and frictional noise, PA1 the heat resistance.
In detail, this means:
The coefficient of friction .mu. should be as high as possible and have very little dependence on the operating and ambient conditions. Since the forces which can be transmitted are proportional to the product of coefficient of friction and contact pressure (.mu..times.F.sub.c), a clutch can, for a given contact force, be made smaller, the higher the coefficient of friction .mu.. The coefficient of friction of the friction lining is dependent on the countermaterial. Customary counter-materials in motor vehicle clutches are grey cast iron or steel. The friction linings used at present for passenger car clutches have, in practical operation, .mu. values of about 0.2-0.4 against grey cast iron. In the design of the clutch, a design value of 0.25 is assumed, which is compensated by safety factors. The coefficient of friction should be as constant as possible with regard to changes in temperature, humidity, contact pressure and angular velocity.
The wear of the friction lining should be low. A guideline for the life of the linings when the clutch is operated correctly is the working life of the motor vehicle. Of course, the countermaterial to the friction lining should not be worn too much by the lining. A measure of the wear is the wear rate.
A high mechanical strength and fracture toughness are required, in particular, when installing the linings by means of riveting. The rotational speed at which rupture occurs should be 1.7-2 times the maximum rotational speed of the engine, i.e. usually 15,000/min, since such high rotational speeds of the clutch disc can occur when mistakes are made in changing down, for example when changing from 5th to 1st gear in a passenger car.
Bucking is among the most serious clutch-related quality problems. Bucking is said to occur when the first resonant frequency of the drive train is excited so strongly that longitudinal vibrations of the vehicle can be felt. Bucking is not easily quantifiable, but is of great importance to the subjective impression of the driver. The intensity of bucking is dependent not only on the lining, but also depends on the structural design of the overall clutch system and its spring elements, although lining-related bucking makes up a significant proportion. Clutches which have no bucking tendency, or have only a limited bucking tendency, are said to have good comfort performance.
In practical use, friction linings of clutches have to withstand considerable thermal stresses without damage, i.e. including no distortion. The aim is to push the temperature limit at which fading occurs further upwards.
A further aspect, namely the environmental friendliness of the materials used, can be regarded as having already been solved, at least insofar as the use of asbestos which was formally customary for clutch linings is no longer necessary.
The presently commercially available friction linings for motor vehicle clutches are organic composite materials. They comprise yarns which are embedded in an frictional cement made of resins, rubbers and fillers such as carbon black, graphite and kaolin. The yarns preferably consist of polyacrylonitrile, aramid, cellulose, glass and other fibres and brass or copper wire. Such friction linings display a good comfort performance but, owing to their organic constituents, in particular the organic matrix, have an unsatisfactory heat resistance, so that when certain temperature limits are exceeded during operation of the clutch the .mu. value drops, leading to a considerable reduction in comfort due to "lining grab" and finally to fading (slipping of the clutch). Further thermal stresses lead to destruction of the friction lining and to complete failure of clutch function.
For some commercial vehicles, e.g. refuse lorries, sintered metal linings based on copper are used. Sintered linings have a high coefficient of friction (about 0.3-0.6) and low wear, but they cause considerable wear of the countersurfaces. In terms of meterability, bucking and frictional noise, metal linings are inferior to the organic linings.
EP 0 469 464 B1 discloses a composite material for friction linings in which the binder matrix is produced from a mixture of SiO.sub.2 and at least some water-soluble silicates, e.g. alkali metal silicates (water glass) which is cured in the presence of water. Such a material will have unsatisfactory hydrolytic stability which is disadvantageous, e.g. if condensed water is formed when the temperature drops below the dew point.
U.S. Pat. No. 4,341,840 discloses glasses reinforced with graphite fibres for bearings, seals and brakes. These composite materials will not be sufficiently heat resistant and will have an unsatisfactory comfort performance.
It is an object of the invention to find a friction lining for friction clutches, in particular motor vehicle clutches, which has an improved property profile, in particular increased heat resistance and thus the ability to transmit a higher torque at a given lining area.
Upon further study of the specification to appended claims, other objects and advantages of the invention will become apparent.